1. Field of the Invention
This invention relates to an electromagnetic flowmeter which detects flow rate of conductive fluid flowing through a process control apparatus.
2. Description of the Prior Art
In an electromagnetic flowmeter, a principle is utilized that an output signal corresponding to flow rate of the fluid can be obtained by sampling electrical potential with a predetermined timing, which is generated in electrodes submerged in the conductive fluid flowing in a magnetic field.
Although electrochemical noise superimposes on the signal detected by the electromagnetic flowmeter, the noise is eliminated by switching polarities of exciting current. As such switching requires a predetermined time, it takes same length of time to stabilize the output performance of the electromagnetic flowmeter. Therefore, the electromagnetic flowmeter has to wait for the stabilization before it executes the sampling in order to obtain the detected signal.
Thus, the electromagnetic flowmeter requires timing which indicates the switching of polarities of exciting current and the sampling. This timing is provided by furnishing the electromagnetic flowmeter with a timing generation means. There is, however, other kinds of of noise besides the above electrochemical noise (as most of these incoming noise are caused by a commercial power supply, these noise will be mentioned as "commercial power supply noise" hereinafter except when it is necessary to distinguish the kind of noise). When the commercial power supply noise is weak, it does not interfere with the performance of the electromagnetic flowmeter. However, since the commercial power supply noise does not syncronize with the timing signal produced by the electromagnetic flowmeter, fluctuation or fluttering of the output signal distinctly appears, and moreover, a surge of the output signal, so called "beat" might be occasionally caused when an amplitude of the commercial power supply noise is great. The performance of the flowmeter can be interfered by the beat.
In the case of apparatus (four-wired electromagnetic flowmeter) driven by the commercial power supply, the apparatus can obtain the timing signal from the commercial power supply itself. Therefore, the timing signal syncronizes with the commercial power supply noise in this case, where a noise reduction effect can be expected. However, even in this case, it is actual status that such an effect as expected can not be obtained since in most of the case the commercial power supply noise from a different commercial power line superimposes on the output signal, too.
Also, in many cases of apparatus (two-wired electromagnetic flowmeter) driven by a DC power supply, fluctuation or fluttering of the output signal is caused since the output signal of the apparatus does not syncronize with the commercial power supply.